1. Field of the invention
The present invention relates to image forming apparatus such as laser printers and more particularly to an arrangement for determining laser beam position.
2. Description of the Related Art
An in-line color laser image printing process where the media passes through four color developing stations in series will print the same speed as monochrome. The photo conductive drum exposures typically occur on all four colors at the same time. Both process and scan direction alignment of the four color developing stations is critical. The process location of each scanning laser beam must overlap to prevent color offset in the process direction. If one beam is out of synchronization with the others, the image in the corresponding color will be misaligned relative to the remaining color components, and the resulting image blurred. Each color should have an adjustment to correct for process direction misalignment because each color has a scanning laser beam following a separate optical path. Thermal changes will cause misalignment so the adjustment should be performed real time and not be just a one time manufacturing adjustment
It is possible to utilize a sensor arrangement for two dimensional beam detection known as PSD or Position Sensitive Detection. In such an arrangement, two optical sensors are used. One functions as a start of scan sensor to detect the horizontal scan beam location. A second sensor may be added at an angle to the first, and the time delay between the two sensors used to determine the vertical beam position location or so-called process direction. Firmware processes this information and makes appropriate adjustments in the raster image. Such an arrangement has a disadvantage in that the second sensor must be longer than the first to achieve the same vertical detection range. If the second sensor is offset by an angle of 45 degrees, the second sensor will be nearly 1.5 times longer than the first, which will be more expensive and may not be feasible.
It is highly desirable to implement an in-line process using four scanning lasers. What is needed in the art is a sensor arrangement which is capable of a full PSD function with reasonable expense.
The invention provides apparatus for determining the coordinates of the location of a laser beam utilizing first and second laser light sensitive sensors which are disposed obliquely to one another.
The invention comprises, in one form thereof, a color printer having multiple laser printheads, each employing a laser beam which executes a scan pattern of repeated transverse scans along a corresponding line to form image information, and laser beam position sensing arrays for each printhead synchronizing operation of the printheads. Each position sensing array includes first and second elongated laser light sensitive sensors extending obliquely to one another and to the corresponding laser scan line.
An advantage of the present invention is that sensor length and cost are reduced, and/or the effective range of process detection is increased.